Screenless Batch Plants Increased Productivity, More Controllability

Mashing big rocks into little rocks. Crushers do this job well, if maintained and operated correctly. The products they give us are well suited to the task of building roads and satisfying the bureaucrats with their specifications. Drum mix AC plants use the stuff ‘as-is’ and don’t seem to suffer unduly. So why do we run this product over a set of screens and disassemble it at our batch plants?

In the old days the answer to that question was simple: It was the most efficient way to ensure a proper blend to meet state specs. But times have changed. With today’s aggregates divided into different stockpiles, screens have become redundant, their value reduced to the removal of oversize material. Worse yet, in some older HMA facilities where the exhaust air and burner systems have been maximized the screens can become a ‘pinch-point’ which actually slows plant production rates through ‘blinding’. Reversing the direction of screen throw sometimes helps combat this situation, but in reality it can’t add substantially to the unit’s flow capacity. The screens can also contribute to ‘out-of-spec’ conditions through carry-over at higher production rates. Another consideration is the amount of time the plant spends waiting for each bin to supply its contribution to the material in the weigh hopper. This wasted time would be best used in mixing-time.

Admittedly, conditions still exist which make screens desirable. A good example would be when a particular plant is switching back and forth between 5/8″ and 3/8″ mixes to supply various customers in the same day. Another would be when a contractor is forced to use 5/8″-0″ rock to produce 3/8″-0″ mix.

Eliminating the screens completely isn’t practical in all cases. So wouldn’t it be nice if we could by-pass the screens when high production rates are desired, yet use them at other times when the need arises?

We can, and it can be done relatively easily. There are two steps we need to take:

1- We must know each feeder’s output and be able to control it precisely.

2- We need to build a mechanical device to divert the aggregates around the screens while maintaining the ability to switch back.

We’ll discuss these steps one at a time. Keep in mind that the purpose of this article is to discuss ONE way to accomplish this task. There are many other ways, each as valid as the next.

Step 1 consists of two parts, modifications for feeder control and the addition of belt scales.

FEEDER CONTROL:

Not so many years ago feeder control meant running out of the control room to the feeders and beating a gate or two up/down to adjust the feed rates at our AC plants. Then we’d race back to the control room, hoping we got things going at the appropriate rates for what we were doing. If we were blending sand we prayed that we didn’t change its ratio enough so that our 200 went out of spec. Crossing our fingers, we chewed Rolaids and waited for the next sample results to see how well we guessed at the gate settings.

It was a great day when management finally had mercy on us and invested in a set of U.S. Vari-drives, or better yet some Woods DC motors and controllers. We were on top of the world. Now we could actually control the rate of aggregate feed from the control-room (no pun intended?).

Most of us were happy as the proverbial clam until the state threw us a curve. New mix specifications decreed that we divide our previously single pile aggregates into three or four separate sizes, depending on what state we were in. 3/4″-0″ became 3/4″-1/2″, 1/2″-1/4″, 1/4″-10 and 10-0″. If we added sand we had five feeders to contend with. We now had to blend these sizes together correctly at the feeders, send them to the batch plant, take them apart over the screens and then mix them back together again in the correct proportions.

Feeder accuracy wasn’t so critical at first. Batch plants have reject chutes. Whatever wasn’t used in the mix was simply discarded, to be carted away and stockpiled for some later use. When fuel and aggregate prices were low this didn’t matter so much, but in today’s market attention to such things can often keep a company operating in the black. As time has progressed it’s become more and more important to precisely control the outputs from the feeders.
Controlling the feeders electrically is the most efficient way to control their outputs. In recent years the prices for electronic gear has seen a downward turn and some exciting new technology has hit the market. Some of this technology can be advantageous in our quest to control the feeders. AC frequency controllers are one example. These devices control motor speed by varying the frequency– normally 60 hertz — of the voltage we send to the feeder motors. If we reduce the hertz to 35 cycles, the motor slows dramatically. Conversely, increasing the frequency speeds the motor up. This allows for an infinite speed range that is repeatable– a must when calibrating your feeders. Older frequency drives had a problem starting motors when set to lower cycles, but the newer drives use ‘full-torque-start’ technology which addresses this problem.

Mitsubishi makes a solid state motor controller for 5 hp motors which sells for under $900 and uses standard electric motors, unlike DC drives which require expensive specialty motors. A standard 5hp TEFC (totally enclosed fan cooled) motor and a TD3 speed reducer serves to drive each feeder nicely. The company also makes units for 10hp motors which sell for around $1400. Square D and Cutler-Hammer offer similar units but at a slightly higher price. These units all utilize existing AC motor leads. Standard start/stop stations also work.

Variable speed feeders alone won’t address everything the batch plant needs to run in a screenless configuration. We must also duplicate the feeder control systems used on drum plants. I.E.: We need to be able to control feeder ratios through the whole range of speeds likely to be used. To do this we must have a way to control each feeder’s speed independently yet still be able to control all linearly, which means each feeder’s percentage of contribution to the overall output must remain constant.

On most jobs we do this by regulating each feeder’s speed with a 20k trim potentiometer hooked to each unit’s 0-10 volt DC reference signal. We then used a signal isolation card, a 2k resistor and a 5k trim pot to control all feeders together. The net result was that we could set our gates and belt speeds to yield a specific rate, then ramp plant speed up or down while maintaining the correct ratios from each feeder. We then added ‘percent’ meters to each feeder for reference points and a fault alarm system which utilizes a piezoelectric horn. A word of caution: Avoid the 200 decibel horn, in a small control room the little devils are painful!

We hooked this system to the customer’s existing 5hp U.S. Vari-drives after locking each drive’s mechanical speed system in the 100% position. As these units fail in the coming years the company will replace them with much cheaper TEFC motors and speed reducers.

Total cost for most jobs is under $5500.00. We are currently upgrading the control circuits and panel to a system featuring manual push-push potentiometers which should make things a little more user-friendly. This will add another $4500.00 to the installation price but should contribute substantially to the ease of operations. Additionally, the new pots will provide a set-point that will be easy to return to as ongoing adjustments are made to feeder ratios.

At this point you are probably questioning the value of such expensive modifications, considering that once you get your batch plant ‘balanced’ you don’t rejecting an overt amount of material. This is a valid point. But the true value of the aforementioned feeder modifications can be easily illustrated: When your plant is running and you have time between trucks take a sample of feeder output. Stop your feeders and conveyors under full-production, I.E. Fully loaded. Take a belt cut sample and run a sieve analysis of it. You will find that it closely resembles the mix design you are using. Variations can almost always be traced to adjustments made to compensate for screen ‘carry-over’ and attempts to compensate for bins rejecting.

A couple of important questions to ask yourself: How long did it take to arrive at a perfect ‘balanced’ condition at the plant? How long will it take to regain that balance when plant speed is increased/decreased? With the feeder modifications discussed earlier, and the addition of a belt scale the answer to both these questions is ‘negligible’. These upgrades are a one-time expense. The time wasted adjusting a plant for efficiency, and the materials rejected in the meantime are an on-going expense incurred every time changes are made. Over time this expenditure will far out-strip the cost of the feeder modifications.

BELT SCALES:

The plant discussed in the feeder section had a Ramsey belt scale and a 10-201 Integrator on it prior to the feeder modifications. The plant’s owner had recognized that belt scales address an issue on a batch plant that cannot be readily addressed any other way: They allow for precise monitoring of the sand to 1/4″-0″ ratio. Since the lion’s share of our aggregate pay factor/penalty comes from this area of our sample results it seems desirable to have as much control of it as possible. Most batch plants use an 1/8″ screen for #1 hot bin. Since sand readily passes through this size, the screens have virtually no effect on the ratio of sand in the mix. No amount of adjusting the batching computer can remedy a problem related to the sand. It can only be addressed at the feeders, and without precise controls, we’re just guessing.

The installation of a set of belt scales is neither excessively expensive nor difficult. They can be installed on your plant’s collecting conveyor– following the manufacturer’s recommendations as to positioning. Or, better yet, they can be installed on the incline conveyor which feeds the dryer. The incline is usually more accessible and easier to work on, making it the logical choice.

Once installed, the scales can provide information formerly unavailable at a batch plant such as actual feed tons-per-hour. To the alert operator they can also indicate those irritating partial feeder blockages that flow alarms won’t catch. Additionally, they can provide information unavailable through any other source, such as an ‘aggregate used’ tonnage at the end of the day. This is used to track actual material wasted at the plant, which can be an eye opener.

Combined with the feeder modifications previously detailed, belt scales bring an unprecedented level of controllability to the batch plant. A discussion of feeder calibration procedures can serve to illustrate this point.

We will use the setup described earlier, installed on a StanSteel RM-80 7000# batch plant, for this discussion.

The first step is to warm-up our belt scales. Start the scale conveyor and allow it to run empty for about 30 minutes. While this is going on we need to figure out the maximum tons per hour we might run. At 45 second batching cycles a 7000# pug-mill will yield 280 tons per hour. Since we don’t want our feeders to limit plant speed, let’s use 350 tons per hour for our maximum rate of feed.

Referencing the mix design for the job we find that we need three aggregates: 5/8″-1/4″, 1/4″-0″ and sand. The ratios are 24%, 70% and 6% respectively.

Armed with these numbers we are ready to proceed. First, set the feeder’s Master Speed Control (MSC) pot to 100%. Then set each individual feeder’s trim pot to 50%. (This will give us room for adjustment later.)

Block off the dryer inlet and provide a path for the aggregate to escape. Verify that you have ‘zero’ on your belt scale integrator. If so, start all the equipment necessary to transport material from the feeders to the dryer. On plants with electrical interlocks it will be necessary to defeat the one which kills the feeders when the dryer stops.

When ready, start the sand feeder. Adjust its gate opening until the belt scale integrator reads 21 tph or as close as possible to it. Record the reading. Stop the feeder and allow the belts to clear. Verify that your belt scale returns to zero, then restart feeder and double check the reading. Repeat this procedure for each feeder. At 245 tph, the 1/4″-0″ will probably require two feeders. Some advice: This is a good time begin getting into the habit of recording every detail of feeder settings so that a particular ‘set-up’ can be repeated at any time.

Once you are finished with calibrating the feeders, it is time to set total tons per hour. Since you can’t start-up at 350 tph, you need to pick a rate which suits your plant and the job you’re mixing for. The RM-80 used as an example throughout this discussion rattled along nicely at 180 tph with little strain, so we used that number as a start-up point. We know that 100% will yield 350 tph, so we need to establish lower reference points to use in our daily operations.

To calibrate total flow: Stop all rock flow and allow belts to clear. Set the MSC pot to 50%. Start all necessary feeders and record tph displayed on belt scale integrator. Turn the MSC pot to 75% and record this reading. Seek the position necessary to yield your chosen start-up tph. When you find it, record the percent on the MSC. Repeat these procedures to verify ‘repeatability’. If all is well you are ready to enjoy the benefits of your new feeder control system. More importantly, your ready to build a screen by-pass to increase your plant’s production rate.

SCREEN BY-PASS ASSEMBLY:

For this discussion we will again use the StanSteel RM-80 7000# batch plant. Other plants may vary in configuration, but the basic idea is the same.

In order to by-pass the screens it is necessary to create a path for the aggregate to travel from the hot stone elevator directly into hot bin #1. On this plant we accomplished this by installing an air controlled gate into the bottom of the feed chute between the hot stone elevator the screens. We then fabricated a chute that entered 1 bin just below the screens, being careful to maintain at least 45 degrees of slope. We lined the gate and both chutes with Scandia steel. Dual air cylinders control the position of the gate which can be operated either manually or automatically.

We added a contact block to the screen motor ‘start’ switch to provide initial power for our solenoid control relay. When the screens are started and the barrel switch is in the auto position, the relay sends power to a ‘gate closed’ indicator light and fires the air solenoid which closes the gate. With the screens off, the gate remains open. When the switch is in the manual position the gate closes and a ‘manual’ indicator light glows alerting the operator that the ‘auto’ system is deactivated.

Once the screen by-pass system is in place we will need to modify one mix menu location in the batching computer so that it pulls all of its aggregate from hot bin #1. Also, some plants use a screen ‘interlock’ which must be defeated to run in a screenless configuration. Additionally, we need some way to control the oversize aggregates that seem to find their way into the most carefully controlled stockpiles. For this job a scalping screen over the incline conveyor at the loading point works well. A 1″ screen will keep your aggregate clean, though a 1.5″ may be required for higher production rates. A vibrating ‘grizzly’ over the drier inlet also works, and costs considerably less. Remember: You are not trying to control aggregate size, only remove errant boulders that have found their way into your feeders.

Assuming that your burner and exhaust air systems have been maximized, you should now be able to run at production rates previously only dreamed of. A 7000# pug-mill at 30 second batch cycles yields 420 tph. If you are dumping into a slat conveyor you can shorten the wet-mix batch timing another 5 seconds and use the conveyor as a mixer. A 7000# pug-mill at 25 second batch cycles yields 504 tph. Both these production rates assume that you are not waiting on the aggregate to weigh up, and while other factors limit production rates the above examples illustrate the potential for increased production. In reality, to run at 30 second or faster batch cycles will probably require additional modifications such as over-sizing the burner and installing a larger main fan, since the real question is how much aggregate we can dry per hour. But by eliminating the screens we have eliminated one ‘artificial’ bottleneck in our HMA facility. The question now becomes: How fast do you want to go?